There are many different approaches that help your rhythm playing, however the one that seems to have helped me the most is doing this:
1. Stick on a metronome
2. Set the speed about 10% slower than you would consider slow.
3 . Try and pick/strum on every click (1,2,3,4)
4. Try and add in an extra strum/pick per click (1, and, 2, and , 3, and, 4, and) while making all the numbers down picks/strokes and all the 'and's as up picks/strokes.
5. Try and add 4 picks per click (1,e,and,a, 2,e,and,a, 3,e,and,a, 4,e,and,a) - This time you would do downstroke for the numbers & the 'and's and upstrokes for the rest.
Practicing this at a slow speed will calibrate your playing to a perfect sense of time, and will allow you to really visualise what's happening with each beat, while working on alternate picking/strumming. It will also force you to learn to hold back to anticipate the clicks, which will make your rhythm playing much tighter
Like Connor Mcgregor said 'Timing beats Speed'